1. Field of the Invention
This invention relates to an improvement applicable to an electric power feeder for a wire electrode type electrical discharge machining apparatus. More specifically, this invention relates to an improvement applicable to an electric power feeder for a wire electrode type electrical discharge machining apparatus, such an improvement having been developed for the purpose of allowing arbitrary selection of any of a plurality of electrical power supply circuits, each having an impedance whose value is different those of the others. The plural electrical power supply circuits are installed to allow a single electrical discharge machining apparatus to realize optimum machining characteristics corresponding to any selected machining speed, required smoothness of the machined surface, etc.
2. Description of the Related Art
It is known that the machining speed decreases for a wire electrode type electrical discharge machining, whenever machining is performed with a more inductive discharge circuit or with a discharge circuit which has a larger value of inductance. Thus, a coaxial cable or the like, which has a relatively small value of inductance, is preferably employed for the discharge circuit of an electrical discharge machining apparatus which is employed for rough machining or preliminary machining. Incidentally, it is known that the smoothness of the machined surface is improved whenever the amount of energy to be applied for each discharge of an electrical discharge machining is smaller. Thus, a discharge circuit having a smaller value of capacitance is preferably employed for an electrical discharge machining apparatus which is employed for precise machining or finish machining.
Since coaxial cables and the like are inevitably involved with a considerable value of stray capacitance, employment of a coaxial cable is not preferable for finish machining performed with an electrical discharge machining apparatus. In other words, the above described two independent requirements are contradictory or antinomical from each other.
Due to the above limitation, a wire electrode type electrical discharge machining apparatus which is desirably employed both for rough or preliminary machining and for precise or finish machining is preferably equipped with "a rough-machining-circuit" having an electric power supply circuit of a coaxial cable or the like and with "a finish-machining-circuit" having an electric power supply circuit of an ordinary cable having a small value of capacitance. The circuits are selectively employed corresponding to the nature of the machining to be conducted.
In the prior art, however, no electric power feeder is available which readily allows selection of the above described two independent electric power supply circuits which are entirely different from each other in the polarity/value of impedance. For example, an electric power feeding having "a circuit-selective-means" consisting of an electromagnetic contactor could be supposed. However, such a feeder would naturally have several drawbacks. The first drawback is that an electromagnetic contactor has a large value of inductance, resultantly causing it to be unqualified for rough machining. The second drawback is that an electromagnetic contactor has an electromagnetic coil, resultantly reducing the reliability of the electrical power supply circuit which is employed for an electrical discharge machining apparatus which employs water as a machining liquid.